1. Field of the Invention
The invention relates to a receiving part for receiving a rod for coupling the rod to a bone anchoring element and a bone anchoring device with such a receiving part. The head of the bone anchoring element is locked in the receiving part by compression of a head receiving portion of the receiving part laterally surrounding the head by means of a locking ring. The receiving part further comprises a pre-locking ring for clamping the head by the pre-stress exerted onto the head receiving portion for safe handling during surgery. The locking ring is movable to a final locking position for locking the head. The bone anchoring device can be realized, for example, in the form of a polyaxial bone screw.
2. Description of Related Art
Various designs of polyaxial bone screws wherein the head is clamped from the side to lock the rotational position of the bone screw are known.
U.S. Pat. No. 5,672,176 describes a bone screw with a receiving part with a conically shaped seat and a conically shaped pressure element which exerts pressure onto the head from above and from the side. If the cone angle has a value laying within a specific range self-locking of the pressure element within the receiving part takes place which allows to preliminary lock the head within the receiving part while the rod is still movable in order to allow the adjustment of its position.
U.S. Pat. No. 5,728,098 describes a bone screw for connection to a spinal rod comprising a screw element and a receiver member which has slits provided at the bottom of the rod receiving channel and wherein two ring-shaped compression members made of a shaped-memory alloy are provided at the lower side and the upper side of the receiver member, respectively. The compression members contract about the portions of the receiver member when the temperature is elevated so that the rod is clamped in the channel.
WO 2007/038350 A2 discloses an apparatus for connecting a bone anchor to a support rod, the apparatus including a connector body and a cap. The connector body has a socket for insertion, angulation and removal of a bone anchor. A sleeve is provided which is configured to fit over the connector body in a temporary position in which the sleeve permits insertion of the bone anchor, to move to a provisional locking position in which the sleeve permits angulation but prevents removal of the bone anchor, and to move to a locking position in which the sleeve prevents both angulation and removal of the bone anchor. The sleeve extends over the whole length of the socket.